API v2 get global task

[c8y3]

Implement endpoint GET /api/v2/global-tasks/{identifier} to get a global task.

• return 200 in case of success
• return a field in case of success
• return 404 when the the global task is not found
• create method in the business layer
• check returned object is uniform with other endpoints
• deprecate GET /global/tasks/<int:cur_id>
• openapi: document endpoint